I know 2011 seems like a million years ago considering what has happened in 2012 so far, but these are the first comprehensive numbers that show just how big e-books grew last year. The report comes from BookStats, a joint venture of the Association of American Publishers and the Book Industry Study Group; BookStats gets its data from nearly 2,000 U.S. publishers across the four major branches of publishing: trade (all the books directed toward regular consumers), K-12, higher education, and professional and scholarly publishing.

I generally focus on what's going on in trade publishing. Let me put it into perspective for you:

-- In 2010, the trade publishing industry had $13.90 billion in revenues and of that, $869 million was from e-books, or about 6% of the total

-- In 2011, overall trade revenues barely moved to $13.97 billion, an increase of 0.5%, while e-book revenues jumped to $2.07 billion, or about 15% of the total

That means that while the trade publishing industry was basically flat in 2011, over $1 billion in revenue ($1.2 billion, to be exact) shifted from print books to e-books.

The number $2 billion in and of itself is not a significant figure any more than its already huge size indicates. What will be significant is when the e-book revenue is high enough to force fundamental changes in the business (more than it already has). Once e-book revenue reaches, say, 50% of total trade publishing revenue, that will mean that a lot of support businesses like packagers, distributors and, of course, bricks-and-mortar retailers will have a different paradigm under which to operate -- not to mention the publishers themselves.

Things are changing, that's clear. The question is, at what pace will they continue to change?

The people I spoke with at the AAP about BookStats wouldn't guess whether e-books were going to double again to $4 billion in 2012, grabbing a third or so of market-share. What other reports have seemed to indicate is that growth in e-books has hit an inflection point and is slowing somewhat as the market becomes saturated. The explosive growth of tablet computers, however, suggests that growth in e-book sales could continue as massive companies like Amazon, Apple and Google continue to emphasize digital content on their tablet platforms (even though tablet owners may read fewer e-books than e-reader owners).

So, it's anyone's guess what the 2013 BookStats report will say, but one thing is nearly certain: E-books will continue to grow in 2013. How much? Your guess is as good as mine.

(For sport, let's guess. I'm going to say that e-books will in fact nearly double in 2012 to $3.9 billion. What do you think? Leave your guess in the comments below.)